Our present invention relates to a fuel line for a motor vehicle and, more particularly, to a layered fuel line of a coextruded type and particularly a type in which the fuel line has an outer layer of polyamide 12 (nylon 12), an intermediate layer of an ethylene-vinyl alcohol copolymer and an inner layer of a polyamide (nylon), for example polyamide 6 (nylon 6).
German patent document DE 40 01 125 C1 describes a fuel line for a motor vehicle of the type in which the intermediate layer serves as a hydrocarbon permeation barrier and thus prevents the diffusion of hydrocarbons from the fuel to the outer layer.
The interior of the fuel line has been found, in motor vehicle applications, to become electrostatically charged to an impermissible level. In addition, the inner part of the earlier layered fuel line tubing is not resistant to ZnCl2 which can form on zinc-containing and usually die-cast parts. With the earlier tubing, when it was assembled to zinc-containing fittings, damage to the tubing could occur, the aging effect was more pronounced and stress corrosion could arise.
Fuel lines of this type are also described in U.S. Pat. No. 5,076,329 which corresponds in part to DE 40 01 125 C1 and in U.S. Pat. Nos. 5,167,259, 5,390,705 and 5,476,120.
Mention may also be made of the teaching in Rxc3x6mpp Chemie Lexikon, Vol. 2, Georg Thieme Verlag Stuttgart, 9th Edition 1990, pages 1110 to 1113, to the effect that certain native solid polymers which are electrically nonconductive can be made conductive by the addition of certain conductive substances like, for example, carbon black or carbon fibers. The resulting electrically conductive polymers, however, are too brittle and insufficiently ductile to be used for the fabrication of coextruded layered tubing and, in particular, tubing for motor vehicle fuel lines.
It is the principal object of the present invention to provide a fuel line for the aforedescribed purposes which is capable of eliminating problems arising from electrostatic charging and a sensitivity to zinc chloride.
Another object of the invention is to provide an improved multilayer fuel piping for motor vehicles which retains its ductility and strength, does not promote corrosion or become corroded readily, even in the presence of zinc-containing fittings, and is free from drawbacks of prior art coextruded tubing.
Still another object of the invention is to provide a low-cost tubing which is resistant to damage by hydrocarbon fuels, does not suffer degradation by diffusion through the tubing and is less sensitive to zinc chloride from fittings with which the tubing is used than has hitherto been the case.
We have found that these objects can be attained when a layer of polyamide 6 constituting the inner layer has up to 30 weight percent of conductive carbon black or graphite fibrils added thereto so as to render such layer electrically conductive and zinc chloride resistant. When that inner layer is polyamide 12, it can also be rendered conductive by the addition of at most 30 weight percent conductive carbon black or graphite fibrils. In particular, the multilayer tubing for conveying a liquid, according to the invention, and especially fuel for a motor vehicle, can comprise:
an outer layer composed of nylon 12;
an intermediate layer coextruded with the outer layer and composed of an ethylene-vinyl alcohol copolymer; and
an electrically conductive inner layer coextruded with the outer and intermediate layer and composed of a nylon selected from the group which consists of nylon 6 and nylon 12 and compounded with an amount effective to render the inner layer electrically conductive and at most 30 percent by weight of at least one conductive substance selected from the group which consists of conductive carbon black and graphite fibrils.
Preferably the tubing forms a motor-vehicle fuel line, the inner layer contains at least 0.1 percent by weight of the conductive substance and sufficient to improve resistance of the nylon of the inner layer to ZnCl2.
The invention is based upon the discovery that, by the addition of at most 30% by weight of conductive carbon black or graphite fibers or fibrils to polyamide 6 (nylon 6) it is possible to make the inner layer composed of nylon 6 sufficiently conductive and also sufficiently resistant to ZnCl2 to enable the use of the tubing to be highly effective for gasoline and diesel fuel for an automotive vehicle. The additives which make the inner layer conductive have been found, surprisingly, to also contribute resistance to ZnCl2.
The invention is applicable not only to the nylon 6 but also the nylon 12 which does have, in the native state and prior to the addition of the conductive carbon black and/or graphite fibrils, a certain ZnCl2 resistance.
According to a feature of the invention, between the outer layer and the intermediate layer of ethylene-vinyl alcohol copolymer and/or between this intermediate layer and the inner layer, respective additional layers of nylon 6 or nylon 12 can be provided which have also been rendered electrically conductive and ZnCl2 resistant by the addition of at most 30% by weight of the conductive carbon black and/or the graphite fibrils. The polyamide of the layer between the outer and intermediate layers and between the intermediate and inner layers can be the same polyamide as that of the inner layer or an equivalent polyamide compound. For example, it can be polyamide 6 or a polyamide 6 compound (PA-6C).
When the polymers of the respective layers bond poorly to one another, bonding layers or polymers with an adhesive bonding characteristic or bonding promotion characteristics can be provided between them.
For example, such a bonding layer (see the aforementioned patents) can be provided between the outer layer and the next closest inner layer, i.e. the intermediate layer or a layer between the outer layer and the intermediate layer or between the inner layer and the next closest polymer layer, namely, the intermediate layer or the additional layer between that intermediate layer and the inner layer.
A suitable bonding promotion layer can be composed of polyolefins having one or more groups and especially polyethylene or polypropylene based compounds. When polyamide 12 is used for the inner layer, this layer can adjoin a bonding layer and any nylon 12 additional layer can also be in contact with one or two bonding layers and an additional layer of nylon 12 can be provided which is not electrically conductive.
The overall wall thickness of the coextruded tubing can be between 0.8 and 2.0 mm and preferably 30 to 60% of the total wall thickness can be constituted by the portion of the tubing lying outwardly of the intermediate layer. A wall thickness of the intermediate layer can be 5 to 25% of the total wall thickness and the portion of the tubing inwardly of the intermediate layer should make up 25 to 55% of the total wall thickness.